The starter ocean

Abstract

Abstract This chapter explores the structure and composition of the ocean floor and how the Paleocene volcanic activity in the Hebrides reflects the attempt to open up a new spreading ridge ‘oceanic’ plate boundary. A series of concentrated igneous dyke swarms at Skye, Mull, Arran and in Antrim together acted as a southwesterly plate boundary diversion to the pre-existing NE-SW band of strike-slip faulting that existed before 60 million years ago, separating Greenland from the continental margin of northwest Europe. The combination of volcanic magma chambers, dyke swarms and lava-fields means that the crust of Skye can be said to have become one third oceanic. These separate spreading ridges, each with 2-3 km of dilation are linked by strike-slip ‘transform faults’ :a fault style first identified by Canadian geophysicist John Tuzo Wilson in the 1960s. Magma in the igneous dykes propagated sideways from magma chambers beneath central volcanoes, as observed in several eruptions on Iceland over the past forty years. Once the magma in the dyke broke through to the surface the dyke would not grow any longer. While standard dykes are 30-50km long and a metre wide, for a period the volcanoes produced giant dykes, that could be ten times as long and ten times or more wide, propagated to the southeast across north Wales and northern England and far across the southern North Sea. These giant dykes, sourced principally from the Mull and Slieve Gullion volcanoes reflect an attempt to create a new plate boundary cutting Britain in two, but only succeeded in splitting the crust by about 200 metres. It took many tens of cubic kilometres of magma to fill a giant dyke, requiring an enormous magma chamber, far larger than any identified in recent eruptions worldwide.